The present invention relates to the preparation of methylpolysilanes having controllable rheologies, and more particularly to a method of preparing such methylpolysilanes by a catalyzed redistribution of alkoxydisilanes, and the methylpolysilanes produced thereby.
In recent years, workers in the art have developed procedures for the preparation of silicon carbide ceramic materials from polymeric silane precursors such as methylpolysilanes. Silicon carbide possesses a number of desirable properties such as chemical inertness, semiconducting properties, extreme hardness and stability at very high temperatures. Accordingly, silicon carbide ceramics have found use in electrical heating units, furnace walls, mufflers, abrasives, rocket nozzles, and automotive and turbine engine parts. Further, it has been found that the use of polymeric precursors permits the formation of fibers and thin films or coatings of silicon carbide which were heretofore extremely difficult to form using inorganic sources of silicon carbide.
Where the ceramic precursor polymers are to be spun into fibers, it is highly desirable that such polymers have rheology characteristics which make them amenable to conventional dry or melt fiber spinning technologies. While the molecular weight of a polymer will have some effect on its suitability for spinning, the glass transition temperature of the polymer is a strong indicator of the suitability of a given polymer for spinning into fibers. Generally, it is desirable that a polymer which is to be spun have a glass transition temperature in the range of 50.degree. to 200.degree. C. and most preferably from 70.degree. to 150.degree. C.
Baney et al, U.S. Pat. No. 4,310,651, teach a procedure for the preparation of methylpolysilanes having halogen substituents through a catalyzed redistribution reaction utilizing tetrabutylphosphonium chloride as the catalyst. The Baney et al process has the advantage of being able to utilize as a starting material the process residue from the direct synthesis of organochlorosilanes. Direct synthesis of organochlorosilanes involves passing the vapor of an organic chloride over heated silicon and a catalyst. See, Eaborn, Organosilicon Compounds, Butterworths Scientific Publications. 1960, page 1. This residue contains a mixture of di-, tri-, and tetra-substituted halodisilanes.
However, the halogen substituents on the methylpolysilanes of the Baney et al process have resulted in some difficulties in handling the compositions which tend to auto-ignite when exposed to oxygen or moisture. Moreover, pyrolysis of the compositions releases large quantities of corrosive HCl or HBr gases which must be handled and properly disposed of. Additionally, the methylpolysilanes of the Baney et al process result in polymers having a fixed glass transition temperature. That is, depending upon the starting materials, each of those polymers have a certain glass transition temperature which cannot be modified to make the polymers more amenable to dry or melt spinning procedures.
Baney et al, U.S. Pat. No. 4,298,558, teach an improved procedure which converts the halogen substituents on the methylpolysilanes to alkoxy or phenoxy substituents. However, the improved procedure still requires a two step process of converting halodisilanes to halo-substituted methylpolysilanes and then converting the halogen substituents to alkoxy or phenoxy-substituted compositions. However, the derivatization of these polymers does not provide an effective method for modifying or controlling their glass transition temperatures.
Other workers have attempted to produce methylpolysilanes by a single step redistribution reaction using methoxydisilane starting materials. For example, Ryan et al, 84 J. Amer.Chem.Soc. 4730 (1962), reported the redistribution of 1,1,2,2-tetramethoxy-1,2-dimethyldisilane to higher polysilanes in the presence of sodium metal. Watanabe et al, in a series of published reports, taught that metal alkoxide catalysts could be used in the redistribution reaction. See, e.g., Watanabe et al, J.C.S. Chem. Comm. (1977) 534; Watanabe et al, J.C.S. Chem. Comm. (1977) 704; Watanabe et al, 128 J. Organometallic Chem. 173 (1977); Watanabe et al, J.C.S. Chem. Comm.. (1978) 1029; Watanabe et al, 218 J. Organometallic Chem. 27 (1981); and Watanabe et al, 244 J. Organometallic Chem. 329 (1983).
Atwell et al, 7 J. Organometallic Chem. 71 (1967), have also reported the redistribution of alkoxy disilanes to higher organopolysilanes. However, in the Watanabe and Atwell reports, the higher organopolysilane was either uncharacterized, unidentified, or was of a low molecular weight (less than 6 silicon atoms in the chain).
More recently, Frey et al, U.S. Pat. No. 4,667,046 teach a method for preparing higher molecular weight methylpolysilanes by reacting a trialkoxysubstituted disilane, and optionally a tetraalkoxysubstituted disilane, with a silane having at least one silicon to hydrogen bond in the presence of an alkali metal alkoxide catalyst. The methylpolysilanes are taught to be useful as negative photoresist coatings and ceramic precursors. However, Frey does not teach the ability to modify or control the rheology characteristics of the methylpolysilanes so produced.
Accordingly, the need still exists in the art for a process for the preparation of ceramic precursor polymers which have controllable rheologies, including controllable glass transition temperatures.